The present invention relates to a preamplifier for amplifying, to the desired level, electric signals generated by transducers such as photoelectric conversion elements and magnetoelectric conversion elements. In another aspect, the invention also relates to a waveform shaping circuit incorporating the inventive preamplifier.
In many cases, electric signals, such as those in electric current and voltage generated by the above-mentioned transducers, are weak. If such signals are transmitted as they are, their signal-to-noise (S/N) ratio will be unsatisfactory. Therefore, it is known to amplify such signals to a desired level by means of a preamplifier, and then to feed them to a signal processing circuit such as a main amplifier, a control circuit, or the like, via a signal transmission line. Accordingly, it is required for a preamplifier to have such characteristics as an excellent signal-to-noise ratio and a high gain.
With a view to satisfying the foregoing conditions, various preamplifiers have been proposed. For example, a preamplifier including an automatic gain control circuit, an output circuit, and a negative feedback circuit has been described in IEEE JOURNAL OF SOLID-STATE CIRCUITS, Vol. SC-21, Apr. 1986, pp 324-330. That preamplifier is capable of controlling gain to a desired level, and in addition has improved frequency characteristics using negative feedback.
However, the preamplifier mentioned above has the following problems. The preamplifier has a complicated overall circuit construction because it needs a control signal generating device for performing gain control, an adjusting device for adjusting the control signals, and so forth. Moreover, since the preamplifier does not have any way of setting an operating frequency in a desired frequency range, nor any way of setting a desired gain and phase, it is difficult to eliminate noise components in a given frequency range, or to eliminate small-amplitude noise components.
Furthermore, the above-mentioned preamplifier is not capable of amplifying the input signals fed into it at a given frequency or in a given amplitude by a desired gain or in an arbitrarily selected phase. In addition, such a preamplifier suffers from decreased gain because its negative feedback circuit is composed of negative feedback resistors.
In another aspect, the present invention relates to a waveform shaping circuit which converts small input current into digital signals at the TTL level, and more particularly to a waveform shaping circuit incorporated in a one-chip integrated circuit on one semiconductor board.
Various proposals have been made for converting input current into digital voltage signals. One example is disclosed in the article, "A Method of Constructing an Integrated High Speed Optical Reception Circuit," Proceedings of General National Assembly of Electronic Communication Society--1984, No. 2638. pp. 10-334.
Specifically, the method presented therein consists in: 1) obtaining the output signal via a branch circuit after the input current fed from a photoelectric converting element is amplified by a plurality of amplifiers, including a preamplifier; and 2) providing a switching circuit for performing the switching of the input dynamic range, an AGC feedback circuit a DC feedback circuit, etc. for controlling the gain in the amplifying system.
The above-mentioned method results in complicated circuit construction, because the method requires various types of feedback circuits, switching circuits, and so forth in addition to the circuits for the amplifying system. Moreover, with such circuit construction, it is not easy to control fluctuations in temperature and current, nor is it easy to compensate for noise. Furthermore, the method makes it inconvenient to install the system, because the analog signal processing system and the digital signal processing system therein are incorporated into separate integrated circuits.
The present inventors have paid particularly keen attention to the accomplishment of the following technical tasks in the course of their development of a waveform shaping circuit which generates digital output signals from input current: 1) providing simple circuit construction for easy incorporation in an integrated circuit, desirably in a one-chip integrated circuit, in order to minimize unfavorable influence due to external factors and, above all, to enable convenient installation of the circuit; 2) achieving stable operation of the circuit with almost no influence from fluctuations in temperature and the current; 3) providing a circuit capable of converting small input current, on the order of approximately several .mu.A into arbitrarily desired digital signals--for example, into signals approximately at the TTL level; 4) providing a circuit capable of generating digital voltage signals in a certain amplitude range even if there are fluctuations in input current and amplifier gain; and 5) providing a circuit capable of performing stable operation against noise.